In the gas turbine engine field, rotor stage assemblies are typically formed of a rotor disk and a plurality of rotor blades extending outwardly from the disk. The rotor stage assemblies are interdigitated between adjacent stator assemblies. An annular flow path for hot working medium gases extends through the rotor stage assemblies and the stator assemblies. Each stator assembly has airfoils which cooperate with the airfoils of the adjacent rotor blades to enable the rotor blades to efficiently remove energy from the hot working medium gases flowing through the assemblies. As a result of the intimate contact between the airfoils of the rotor blades and the hot working medium gases, heat is transferred from the hot gases to the rotor blades.
In modern aircraft engines, cooling air may be flowed through each blade to remove a portion of this heat from the blade, reducing the temperature level and spanwise gradients in the blades and thereby improving the service life of the blade. An example of such a coolable rotor blade is shown in U.S. Pat. No. 3,635,586 to Kent et al entitled "Method and Apparatus for Turbine Blade Cooling".
Heat transfer from the hot working medium gases to the rotor disk is also of concern. The hot working medium gases may locally heat the rim region of the disk causing thermal gradients and stresses which decrease the service life of the disk. One approach to solving this problem is to cool the face of the disk with jets of cooling air. An example of such a construction is shown in U.S. Pat. No. 2,858,101 to Alford. In Alford, cooling air is discharged in jets through a metering nozzle onto the face of the disk. The metering nozzles are oriented in a direction opposite to the direction of rotation of the disk.
Another approach is to flow purge air through a cavity between a rotor and stator structure, inwardly of the working medium flow path, to prevent the ingestion of hot gases from the working medium flow path. Work must be expended by the rotating machinery of the engine, such as a compressor, to pressurize the purge air. Accordingly, a loss of the purge air into the flow path decreases the efficiency of the engine.
The loss of purge air is increased by a large boundary layer between the purge air and the rotating rotor disk as compared with construction having a small boundary layer. The rotor disk acts as a centrifuge and through rotational forces pumps radially outwardly the air in the boundary layer. The flow outwardly of purge air may be balanced by the flow into the cavity of additional purge air, further decreasing the efficiency of the engine. Alternatively the purge air pumped from the boundary layer may be replaced by hot working medium gases from the flow path which are mixed with the purge air and recirculated to the gas path. Recirculating flow from the gas path into the boundary layer and back to the gas path causes deleterious heating of the rotor disk and also decreases the efficiency of the engine.
Accordingly, scientists and engineers continue to seek improved cooling systems for rotor assemblies which have minimal adverse effect upon the efficiency of the operating engines and yet provide satisfactory cooling of the rotor components.